1. Field of the Invention
The present invention relates to a beam control device in an exposure device incorporated into a digital copying apparatus or the like, the beam control device controlling a light beam.
2. Description of the Related Art
An exposure device is known which is applicable to, for example, an image forming apparatus such as a digital copying apparatus and which exposes a charged area of a photosensitive drum to a light beam such as a laser beam.
The exposure device has a semiconductor laser element that emits a laser beam, an optical member that sets the diameter of the laser beam emitted by the semiconductor laser element, at a value corresponding to a desired resolution, a polarizing device that polarizes the laser beam so that the beam travels in a main scanning direction (along an axial direction of the photosensitive drum), an image forming lens system that forms the laser beam polarized by the polarizing device into a substantially linear image at a predetermined position on the photosensitive drum, and other components.
Further, the exposure device comprises an application specific integrated circuit (ASIC) and a central processing unit (CPU) acting as a control section. For example, the ASIC converts image data read by a scanner or the like into image data that can be used to drive the semiconductor laser element. Then, the intensity of the laser beam emitted by the semiconductor laser element is changed on the basis of the converted image data.
The laser beam is polarized by the polarizing device and is partly detected by a horizontal synchronizing sensor. The laser beam detected by the sensor yields a horizontal synchronizing signal. The ASIC changes the intensity of the laser beam outputted by the semiconductor laser element, according to the image data read by the scanner or the like. In this manner, on the basis of the horizontal synchronizing signal generated by the horizontal synchronizing sensor, the intensity of the laser beam is changed according to an density of image data corresponding to one line in the main scanning direction. Then, in many exposure devices, when an instruction to form an image, i.e. to start exposure is given, a predetermined control signal is supplied to the ASIC and CPU. Then, the ASIC first outputs control image data (or a reference signal) used to change the intensity of the laser beam outputted by the semiconductor laser element, according to the image data read by the scanner or the like, the laser beam being outputted on the basis of the read image data. Accordingly, if the ASIC and the CPU are activated correctly, the semiconductor laser element emits the laser beam having its intensity changed so as to correspond to the control image data.
Whether or not the ASIC has been activated normally can be determined depending on whether or not the horizontal synchronizing sensor is outputting a horizontal synchronizing signal indicating that the laser beam emitted by the semiconductor laser element has been detected.
If the horizontal synchronizing sensor is outputting a horizontal synchronizing signal, the ASIC supplies control image data to the semiconductor laser element to cause the semiconductor laser element to output a predetermined laser beam.
In contrast, if the ASIC is not activated correctly, the semiconductor laser element does not output any laser beams having their intensities changed so as to correspond to control image data. Consequently, the horizontal synchronizing sensor does not output any horizontal synchronizing signals. In this case, the CPU determines that a laser driving circuit is abnormal or that an error has occurred during the activation of the ASIC.
However, noise or the like may hinder the ASIC from timely supplying control image data to the semiconductor laser element even though the ASIC has been activated normally. As a result, the semiconductor laser element may fail to output a laser beam corresponding to the control image data (even if it is outputted, the timing for outputting may be later than a predetermined one).
In this case, even if the ASIC is activated correctly, the CPU disadvantageously detects an error to stop the subsequent operations.